In order to form an image in an image forming apparatus of the electrophotographic system, an electrostatic latent image is first formed on a photosensitive member. The electrostatic latent image is then developed with a developer (toner) on a developing roll to form a toner image. This toner image on the photosensitive member is transferred to any of various recording material such as paper and OHP sheets as needed. The toner image transferred is fixed thereto by a system of heating, pressing, solvent vapor or the like, whereby a printed image is formed. The toner remaining on the photosensitive member without being transferred to the recording material upon the transfer is recovered by a cleaning step. As a cleaning method, a method, in which a cleaning blade is brought into contact with the surface of the photosensitive member to remove the remaining toner, is widely used in that the device is simple.
In recent years, image forming apparatus of the electrophotographic system have been spread over offices, and a proportion of full-color image forming apparatus to monochromatic image forming apparatus has been recently increased. With the trend to full-color image forming apparatus, high-quality toners having various properties such as better image reproducibility, durability upon printing and environmental stability are required for toners (generally 4 color toners of cyan, magenta, yellow and black) used in development.
From the viewpoint of image reproducibility among the properties required, it is effective that toner particles are formed as particles having a spherical form, a small particle diameter and a narrow particle diameter distribution. As a process for producing such toner particles, there have been polymerization processes such as a suspension polymerization process, in which a polymerizable monomer composition containing a polymerizable monomer, a colorant and various additives is formed into droplets in an aqueous medium and then polymerized to form colored polymer particles; and an emulsion polymerization aggregation process, in which a polymerizable monomer emulsified is polymerized to synthesize fine binder resin particles, and the fine binder resin particles are then aggregated together with fine colorant particles to produce toner particles.
However, the toner particles having a spherical form and a small particle diameter have involved a problem that when the cleaning method making use of a cleaning blade is applied to the toner particles, cleaning failure is easy to occur. As a method for improving the cleaning ability of the resulting toner particles, a method for controlling the shape of toner particles has heretofore been investigated.
On the other hand, inorganic photosensitive members of selenium type, silicon type, etc. have been mainly used as photosensitive members, but organic photosensitive members are mainly used at present. The polarity of a photosensitive member charged includes a positively charged system and a negatively charged system, and the negatively charged system is general for the organic photosensitive member. However, the negatively charged system is indicated as involving a problem of ozone generation. Thus, an organic photosensitive member of the positively charged system, which is little in ozone generation, has come to be put to practical use for environmental measure.
Under the foregoing circumstances, there is a demand for development of a high-quality positively charged developer for development of electrostatic images, which meets the requirements attending on the trend to full-color image forming apparatus and can correspond to the positively charged system.
Japanese Patent Application Laid-Open No. 2005-345975 has proposed a positively charged toner containing a resin, a parting agent and colorant particles, wherein the concentration of a cationic surfactant on the surfaces of toner particles is higher than in the central portions of the toner particles. The cationic surfactant is an alkyl group-containing quaternary ammonium salt, and the toner particles are formed by the emulsion polymerization aggregation process. This publication describes that a negatively charged external additive is used as an external additive.
Japanese Patent Application Laid-Open No. 2006-18251 (corresponding to US 2005/0277047 A1) has proposed a positively charged developer for development of electrostatic images, which contains a copolymer comprising an acrylamide having an amine or ammonium salt in its molecule in surface portions (outer layers) of toner particles. This publication describes that a negatively charged external additive is preferred as an external additive.
However, these toners are not sufficient in environmental stability and insufficient in environmental stability in a high-temperature and high-humidity environment in particular, and so more improvement has been desired.